Unsaturated ester compounds, such as unsaturated alkyd resins, particularly the esters of acrylic or methacrylic acid, respectively, and especially the acrylates or methacrylates of poly-functional alcohols, are polymerized by means of substances supplying free radicals, particularly by means of organic peroxides.
The radicals introducing this polymerization of the olefinically unsaturated compounds can also develop by means of ultraviolet radiation in combination with so-called ultraviolet initiators or sensitizers when the unsaturated substances to be polymerized contain such initiators and are then subjected to an intensive radiation by means of ultraviolet light. Benzoin or its ether derivatives, respectively, have been used for a long time as particularly suitable ultraviolet initiators for the unsaturated polyester substances. Photopolymerizable dental substances for tooth fillings consisting of a mixture of polyacrylates and acrylic ester monomers and containing benzoin as photoinitiator are described in the British Pat. No. 569,974 (1945). These substances are hardened by means of ultraviolet radiation in the mouth. However, in practice it has been found that the required periods of time for the radiation necessary to achieve polymerization were too long and, therefore, this procedure was not considered of great importance at that time.
The photo-hardening of substances containing poly-functional acrylic esters is also utilized in other fields. Thus, printing ink or finishing varnish are described in the British Pat. No. 1,198,259 which contain benzoin ethyl ether as ultraviolet initiator and are hardened by means of ultraviolet radiation. Polymer compositions are described in German Offenlegungschrift No. 23, 15, 645 which consist of reaction products of organic isocyanates and hydroxyalkylacrylate and contain unsaturated monomers, such as alkylacrylate or alkylmethacrylate. These substances contain also benzoinalkylether and can be hardened by radiation. In the same way, similar substances are described in the German Offenlegungschrift No. 23, 20, 038 which are utilized in dentistry.
Since unsaturated polyesters often become unstable and tend towards a premature polymerization when the very sensitive polymerization initiators are added so that these substances have then only a short storage time, stabilizers are added to them, e.g. phenolic compounds such as hydroquinone or methoxyphenol. Also organic phosphites, e.g. trimethylphosphite or triphenylphosphite, are disclosed in German Offenlegungschrift No. 19, 34, 637, together with a cuprous salt of an organic acid, for the stabilization of unsaturated polyester substances containing benzoin or its derivatives whereby the phosphite stabilizer should be present in an amount of 200-800 ppm. Ultraviolet-hardenable coloring, impregnating, coating or priming substances based on unsaturated polyesters are described in German Offenlegungschrift No. 21, 04, 958 which contain, besides benzoin ether, also organic esters of the phosphorous acid and organic derivatives of phosphine. In this way, a good storage stability is achieved with a short polymerization speed at the hardening by means of ultraviolet radiation. The utilization of esters of phosphorous acid, e.g., triphenylphosphite, is also described in the German Auslengungschrift No. 10, 98, 712 as an additive to increase the storage and color stability of unsaturated polyester resin substances.
The ultraviolet-initiated hardening of tooth filling substances as well as the sealing of teeth by means of such coating substances has met with great interest in the field of dentistry in the past years. However, so far, either very intensive radiation units must be utilized or long periods of radiation are required in order to obtain a complete hardening. Furthermore, these preparations cannot contain any additives which could effect a discoloring of the polymerized substance after some time under the influence of the environment in the mouth, particularly of the different types of food and liquids. Consequently, the organic phosphine compounds made known, for example, as catalysts of the benzoin-initiated ultraviolet polymerization in the case of the unsaturated polyester substances of German Offenlegungschrift No. 21, 04, 958 cannot be used with dental substances based on acrylic or methacrylic esters because these phosphine compounds lead to a discoloring of the tooth fillings or sealings after a period of time.